1. Field of the Invention
The present invention relates to a labeling machine and, more specifically, to a sheet member impression structure for use in a labeling machine to keep a transferring sheet member smooth.
2. Description of the Related Art
FIG. 1 shows a pair of sheet member impression devices 1a mounted on a rod member 2a and locked by a respective tightening up screw 3a to impart a downward pressure to a pressure board 4a, keeping the sheet member Sa being transferred beneath the pressure board 4a smooth. The pitch between the sheet member impression devices 1a is determined subject to the size of the sheet member 5a. Each sheet member impression device 1a comprises a cylindrical casing 11a, a cap 12a located on the top side of the cylindrical casing 11a, a spring member 13a mounted inside the cylindrical casing 11a, and a pressure tip 14a provided at the bottom side of the spring member 13a and extended out of the bottom side of the cylindrical casing 11a. The cylindrical casing 11a has two axle holes 11a transversely aligned at two sides near the cap 12a and a bottom opening 112a. The diameter of the axle holes 11a fits the outer diameter of the rod member 2a so that the cylindrical casing 11a can be firmly secured to the rod member 2a. The diameter of the bottom opening 112a is smaller than the inner diameter of the cylindrical casing 11a. The pressure tip 14a passes through the bottom opening 112a to the outside of the cylindrical casing 11a. The spring member 13a is adapted to adjust downward pressure of the sheet member impression device 1a. The pressure tip 14a is pressed on the pressure board 4a, controlling smooth transfer of the sheet member 5a. 
The sheet member impression devices 1a are spaced from each other at a distance so that a downward pressure can be evenly applied to the pressure board 4a, keeping the transferring sheet member 5a smooth. When a different size of sheet member is used, the pitch between the two sheet member impression devices 1a must be properly adjusted.
When adjusting the position of each sheet member impression device 1a, a tool, for example, a screwdriver shall be used to unfasten the tightening up screw 3a, for enabling the respective sheet member impression device 1a to be moved along the rod member 2a to the desired position. This pitch adjustment procedure is complicated.
Further, because the contact between each sheet member impression device 1a and the pressure board 4a is a point contact, downward pressure may not be evenly applied from the sheet member impression devices 1a to the pressure board 4a. 
The present invention has been accomplished under the circumstances in view. It is the main object of the present invention to provide a sheet member impression device for labeling machine, which provides a line contact to apply a downward pressure to the pressure board in the labeling machine evenly, keeping the transferring sheet member smooth.
To achieve this and other objects of the present invention, the sheet member impression structure comprises a rod member, and two sheet member impression devices mounted on the rod member and respectively pressed on a pressure board to smooth a sheet member being transferred beneath the pressure board. Each sheet member impression device comprises a casing mounted on the rod member, the casing comprising an inside space, two through holes aligned at two sides in communication with the inside space and adapted to accommodate the rod member, a side opening, and a bottom hole; an adjustment unit mounted in the inside space inside the casing below the rod member and partially extended out of the side opening of the casing for operation by the user to adjust the position of the casing on the rod member; a spring member mounted in the inside space inside the casing below the adjustment unit; and a press member mounted in the inside space inside the casing and supported on a bottom side of the spring member and partially extended out of the bottom hole of the casing and forced by the spring member to press on the pressure board.